Chiroptical molecular propellers based on hexakis(phenylethynyl)benzene through the complexation-induced intramolecular transmission of local point chirality

Article abstract of DOI:10.1039/c4ob01601g

We designed hexakis(phenylethynyl)benzene derivatives with a tertiary amide group on each blade to achieve a helically biased propeller arrangement through the complexation-induced intramolecular transmission of point chirality. A hydrogen-bonding ditopic

Full text of DOI:10.1039/c4ob01601g

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Chiroptical molecular propellers based on
hexakis(phenylethynyl)benzene through the
complexation-induced intramolecular
transmission of local point chirality†
Cite this: DOI: 10.1039/c4ob01601g
a,b
a
a
a
a
Ryo Katoono,* Keiichi Kusaka, Shunsuke Kawai, Yuki Tanaka, Keisuke Hanada,
c
a
a
Tatsuo Nehira, Kenshu Fujiwara and Takanori Suzuki*
We designed hexakis(phenylethynyl)benzene derivatives with a tertiary amide group on each blade to
achieve a helically biased propeller arrangement through the complexation-induced intramolecular trans-
mission of point chirality. A hydrogen-bonding ditopic guest was captured at two amide groups, and thus
could pair two neighboring blades to form a supramolecular cyclic structure, in which an auxiliary chiral
group associated with a blade acted as a chiral handle to control the helical bias, while the chiral auxiliary
did not exert any helical influence on the dynamic helicity in the absence of a guest due to the high flexi-
bility of each blade.
Received 28th July 2014,
Accepted 28th August 2014
DOI: 10.1039/c4ob01601g
www.rsc.org/obc
2
in which six sp carbons of four aryl and two amide blades
were directly connected to the benzene ring as a central core,
Molecular propellers are interesting in terms of helical chir- and the molecules satisfied the above two requirements even
Introduction
1,2
3
ality in addition to actual helices such as polymeric chains
in solution, as well as in the crystal form. We have recently
4
and helicenes. Among them, dynamic molecules that show been studying the framework of hexakis(phenylethynyl)-
2
a,b,7,8
interconversion between conformations with (M)- or (P)-heli- benzenes (HPEBs),
in which six phenyl groups are con-
2
5
city are suitable for the studies on transmission of chirality,
nected to a central benzene ring through a triple bond to give
which is the central issue in the design of functional an extended π-electron system. HPEBs have also provided
molecules, e.g. asymmetric catalysts, sensors, and memory molecular motifs for studies on columnar assemblies by stack-
2
a,7a
9
materials. To investigate and utilize helical chirality based on ing,
including discotic liquid crystals, and unique optical
7
a–e,10
a dynamic molecular propeller, the molecule should be properties based on the large π-plane.
The peripheral
designed to satisfy the following requirements: (1) all blades phenyl rings rotate freely about the single bond and the mole-
around the central core should twist in a conrotatory manner, cule can adopt many different conformations. Among these
and (2) the helical preference should show a particular bias. numerous conformations, only the two conformers in which
However, it is not sufficient to develop a methodology for both all six blades are twisted in a conrotatory manner are regarded
constructing a propeller-shaped molecule and biasing the as chiral propellers (Scheme 1). It is a challenging task to force
dynamic helicity to prefer a particular sense at the molecular
level. Previously, we reported the design of chiroptical mole-
6
cular propellers based on tertiary tetraarylterephthalamides,
a
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo
060-0810, Japan. E-mail: katoono@sci.hokudai.ac.jp, tak@sci.hokudai.ac.jp;
Fax: +81 11 706-2714; Tel: +81 11 706-3396
b
School of Materials Science, Japan Advanced Institute of Science and Technology,
Nomi, Ishikawa 923-1292, Japan
c
Graduate School of Integrated Arts and Sciences, Hiroshima University,
Higashi-Hiroshima 739-8521, Japan
†
Electronic supplementary information (ESI) available: Supplementary figures,
and experimental details of new compound preparation and X-ray analysis.
CCDC 1011177 and 1011992. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4ob01601g
Scheme 1 Diversity in the conformation of hexakis(phenylethynyl)-
benzene (HPEB) including chiral propellers with (M)- or (P)-helicity.
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the molecule to adopt propeller conformations based on the
HPEB motif at the molecular level, and to purposefully control
the dynamic helicity to prefer a particular sense, while there
have been a few reports that a helical conformation helped to
produce enhanced chiroptical signals in a helically stacked
7
a
assembly.
In this study, we designed HPEBs 1 with a tertiary amide
group on each blade (Fig. 1). The chiral auxiliary [(R)-C*HMe-
(cHex)] on the amide nitrogen should provide a local chiral
space in the blade, but should not collaborate with the neigh-
boring blades to force the whole molecule to adopt a helically-
biased propeller arrangement due to the high flexibility of the
peripheral phenyl rings. Therefore, we would need to pair two
neighboring blades to achieve chiral communication between
8
blades. We envisioned that a ditopic guest molecule would
bind at the two amide carbonyls in a pair of neighboring
blades through hydrogen bonds to form a supramolecular
cyclic structure, in which the two blades would be forced to
work in collaboration with each other by twisting in a conro-
tatory manner and the local point chirality would act as a chiral Scheme 2 Generation of a chiroptical propeller through the complexa-
handle to control the direction of twisting to prefer a particular tion-induced intramolecular transmission of point chirality.
sense of (M)- or (P)-helicity. Consequently, the 1 : 3 complexa-
tion of (R,R,R)-1b [X = CH (cHex), Y = (R)-C*HMe(cHex)] or
2
1
,2-bis(phenylethynyl)benzene derivatives that are considered
(
R,R,R,R,R,R)-1c [X = Y = (R)-C*HMe(cHex)] with an achiral
to be one-third of the HPEB framework (Fig. 1), and then apply
this method to the HPEB framework to achieve enhanced chir-
optical signals from (R,R,R)-1b and (R,R,R,R,R,R)-1c in their
complexed states.
ditopic guest would lead to a C -symmetric propeller arrange-
3
ment in the HPEB framework, and the helical preference of
the propeller would be biased by the complexation-induced
intramolecular transmission of local point chirality (R) in each
of the three supramolecular cyclic structures (Scheme 2). We
describe below the details of a method for the complexation-
induced intramolecular transmission of chirality on the basis Results and discussion
of results with double-bladed substructures 2, which are
Molecular design and preparation
We used three combinations of auxiliaries (X and Y) on the
amide nitrogens in HPEBs 1 and the double-bladed substruc-
tures 2 (Fig. 1): neither X nor Y has a chiral carbon for 1a and
8
2
a, only Y has a chiral carbon for (R,R,R)-1b and (R)-2b, and
both X and Y have a chiral carbon for (R,R,R,R,R,R)-1c and
R,R)-2c. D6h-Symmetric 1a and C -symmetric (R,R,R,R,R,R)-1c
were prepared by Sonogashira coupling reactions of hexachloro-
(
6
7
d
benzene with the corresponding phenylacetylenes. Double-
bladed substructures 2a and (R,R)-2c were also obtained by
Sonogashira coupling reactions, and an achiral ditopic hydro-
gen-bonding guest 4, without any chiral element, was prepared
1
2
by acidification of a known 1,4-xylylenediamine, followed by
counter anion exchange to achieve high solubility in organic
media, and used for complexation with 1 or 2 (Scheme S1†).
Molecular structure and spectroscopic characterization of
double-bladed substructures 2
A single-crystal X-ray analysis for 2a demonstrated a helical
conformation with the two phenylethynyl blades twisting in a
conrotatory manner and the two benzoyl groups adopting a
1
3
cis-conformation and facing inwards (Fig. 2). In the crystal,
two conformers with (M)- or (P)-helicity were present in 1 : 1
ratio. In a conformational search for a model 2a′ [X = Y = Me],
Fig. 1 Chemical structures of HPEBs 1, double-bladed 2, single-bladed
, and ditopic guests 4 and 5.
3
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2 1
Fig. 2 X-ray structure of (P)-2a [X = Y = CH (cHex)] (P2 /c, Z = 4) in
rac-2a·chloroform crystal: (a) top view and (b) side view. The crystalliza-
tion solvent is omitted for clarity.
Fig. 4 (a) UV and (b) CD spectra of (R)-2b (thin line), (R,R)-2c (thick
we found that a conformation similar to that seen in the line), and (R)-3 (dashed line), measured in CH
crystal of 2a was the most energy-minimized structure (Fig. 3).
2 2
Cl at room temperature.
1
The H NMR spectrum of 2a showed only one set of aver-
diphenylacetylene derivative with the same chiral auxiliary
(R)-C*HMe(cHex)] on the amide nitrogen. In the spectrum of
R,R)-2c with the same chiral auxiliary attached to each of the
aged resonances, which was assigned to a single C -symmetric
2
v
[
(
species in solution (Fig. S1†). The observed symmetry was
explained by assuming high flexibility in the para-phenylene
and benzoyl groups, involving interconversion between the
two energetically-equivalent helical conformations with (M)- or
two blades, the molar CDs around 290 nm were simply
doubled compared to those of (R)-2b or (R)-3 (Fig. 4b), which
indicated that the local chirality on each blade did not signifi-
cantly induce a preference regarding the double-bladed
dynamic helicity, but presented a local chiral space for each
(
P)-helicity (dynamic helicity). The UV spectra of 2a–c showed
two major absorptions that were assigned to the diphenylacetyl-
ene unit (293 nm for 2a, 291 nm for 2b, and 289 nm for 2c)
and the longer conjugation through the ortho-phenylene group
1
5
blade independently.
1
4
(around 330 nm sh) (Fig. 4a), and the similarity in absorp-
tion suggested that a common structure was present in solu- Complexation-induced intramolecular transmission of local
1
5
tion for the double-bladed substructures 2.
In the UV point chirality
spectrum of single-bladed 3, which is a diphenylacetylene
First, we investigated the 1 : 1 complexation of a double-bladed
substructure 2 with a ditopic guest by H NMR spectroscopy.
derivative, we observed only one set of absorptions around
1
1
4
2
91 nm, as expected (Fig. 4a).
The CD spectrum of (R)-2b mainly showed negatively-
The stoichiometry was confirmed to be 1 : 1 by a Job plot for
11
the complexation of 2a with (R,R)-5 based on a continuous
signed Cotton effects around 290 nm (Fig. 4b), which did not
seem to be due to a helically-biased double-bladed dynamic
helicity, but rather mainly resulted from an interaction
between a benzoyl group and a phenylethynyl blade (weak
chiral communication between blades). This idea was sup-
ported by the fact that quite similar Cotton effects were also
present in the spectrum of single-bladed (R)-3, which is a
change in the chemical shift of 2a (Fig. 5a). We estimated the
Fig. 5 (a) Job plot for the complexation of 2a with (R,R)-5 in CDCl
03 K using continuous changes (Δδ = δ2a(R,R)-5 − δ2a) in the chemical
shift for phenylene protons (close to the amide group) in the blades
[2a] + [5] = 2 mM). Some chemical shifts were not recorded due to
3
at
3
(
Fig. 3 Energy-minimized structure for 2a’ [X = Y = Me] obtained by a
conformational search with the MacroModel software (v9.9 Monte Carlo
Multiple Minimum method, OPLS_2005, non-solvated, 20 000 steps): (a)
top view (space filling representation) and (b) side view (stick represen-
tation). Only one of the enantiomeric conformations with (M)- or
peak-broadening in the region of 0 < χ2a < 0.3; (b) continuous changes
in the CD spectrum of 2a (3.5 × 10− M) upon complexation with a chiral
ditopic guest (R,R)-5 (blue lines; 1, 2, and 4 equiv.) or (S,S)-5 (red lines; 1,
4
2, and 4 equiv.), measured in CH
2 2
Cl at room temperature. Molar CDs
from the chiral guests 5 were very small (Δε < ±0.1) in their absorption
11
(
P)-helicity is depicted.
region.
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association constant (2 × 10 M−1) for the 1 : 1 complexation
through a titration experiment, followed by a curve-fitting
method (Fig. S3a†). Then, we monitored the complexation of
3
2
a, which has no chiral element other than the dynamic
helicity interconverting between (M)- and (P)-helical confor-
mations, with chiral ditopic guests (R,R)-5/(S,S)-5 by CD spec-
troscopy. In the CD spectrum of 2a, we found that bisignated
Cotton effects were induced in the absorption region of 2a
upon the gradual addition of (R,R)-5 into a solution of 2a
[λext(Δε) 287 nm (−6), 343 nm (+2)] (Fig. 5b). Mirror images of
the supramolecularly-induced Cotton effects were obtained
using (S,S)-5 instead of (R,R)-5 (Fig. 5b), and indicated that the
dynamic helicity of 2a was biased to prefer a particular sense
in each of the enantiomeric complexes through the supramole-
−4
Fig. 6 Continuous changes in the CD spectra of (a) (R)-2b (2.6 × 10
M) and (b) (R,R)-2c (2.2 × 10
−4
M), upon complexation with achiral
cular transmission of chirality in a guest to the double-bladed ditopic guest 4 [(i) 0 equiv. (2 only, dashed line), (ii) 1 equiv., (iii) 2 equiv.,
dynamic helicity of the host. These bisignated Cotton effects and (iv) 4 equiv. (solid lines)]. All spectra were measured in CH
2 2
Cl at
room temperature.
were considered to be the same as those seen around at
1
5
3
50 nm in the spectrum of (R)-2b or (R,R)-2c, and are dis-
cussed again in the following experiments.
preference was biased to a particular handedness. This is also
the case for a complex of (R,R)-2c with 4, and the induced
Cotton effects were similar in appearance, and greatly
enhanced (Fig. 6b). As a control experiment to confirm that
induced changes in UV and CD spectra were due to a com-
plexed species through hydrogen bonds, we added acetonitrile
We describe here a method for the complexation-induced
intramolecular transmission of chirality: the local point chiral-
ity associated with a blade is intramolecularly transferred to
the dynamic helicity to act as a chiral handle only in the supra-
molecular cyclic structure formed by complexation, leading to
a biased helicity through the complexation-induced communi-
cation of chirality between blades (Scheme 3). We examined
the complexation of (R)-2b with the achiral ditopic guest 4,
which has no preference for a particular sense, upon the
gradual addition of 4 to a solution of (R)-2b. The Cotton effects
were remarkably changed from the original ones of (R)-2b
itself to show a positive couplet [λext(Δε) 285 nm (−14), 329 nm
16
(
10 wt%) to a solution of (R,R)-2c in CH Cl in the presence of
2 2
4
, and confirmed that no change was induced in both UV and
CD spectra of (R,R)-2c (Fig. S4†).
Molecular structure and spectroscopic characterization of
HPEBs 1, and formation of a chiroptical molecular propeller
(
+5)] (Fig. 6a). This continuous change in the CD spectrum A single-crystal X-ray analysis for 1a showed a centrosymmetric
indicated that the local point chirality associated with one of structure (P1
ˉ, Z = 2), in which four blades were twisted and the
the two blades of (R)-2b was intramolecularly transferred to rest were planar toward the central benzene core (Fig. 7). All
the double-bladed dynamic helicity by the formation of a amide groups adopted a cis-conformation as in the case of 2a.
17
supramolecular cyclic structure to give one-third of a propeller
3
In a conformational search for 1a, a C -symmetric propeller
arrangement based on the HPEB framework, and the helical arrangement was predicted as the most energy-minimized
structure (Fig. 8), and is considered to be composed of a three-
fold helical double-bladed substructure.
1
In the H NMR spectra of 1, only one set of averaged reso-
nances for phenylene protons among all of the HPEBs was
observed (Fig. S1†), and these observed symmetries indicated
that a single species with (pseudo)six-fold symmetry was
Scheme 3 The intramolecular transmission of chirality in a supramole-
cular cyclic structure that was formed by complexation with an achiral
ditopic guest.
Fig. 7 X-ray structure of 1a·H
contained water, which is omitted for clarity.
2
O [X = Y = Me] (P 1¯ , Z = 2). The crystal
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Fig. 8 Energy-minimized structure for 1a [X = Y = Me] obtained by a
conformational search with the MacroModel software (v9.9 Monte Carlo
Multiple Minimum method, OPLS_2005, non-solvated, 50 000 steps).
Only one of the enantiomeric conformations with (M)- or (P)-helicity is
depicted.
Fig. 10 Continuous changes in the CD spectra of (a) (R,R,R)-1b (8.7 ×
−5
−5
10 M) and (b) (R,R,R,R,R,R)-1c (8.1 × 10 M), upon complexation with
achiral ditopic guest 4 [(i) 0 equiv. (1 only, dashed line), (ii) 3 equiv., (iii) 6
equiv., and (iv) 12 equiv. (solid lines)]. All spectra were measured in
present in solution, even for (R,R,R)-1c, which actually reflects CH
C3 symmetry. As was demonstrated for 2, these averaged reso-
nances suggested high flexibility in the para-phenylene and
2 2
Cl at room temperature.
The simple duplication of molar CDs in the CD spectra of
R,R,R)-1b and (R,R,R,R,R,R)-1c was accounted for by the
absence of chiral communication between blades, as in the
case for (R)-2b and (R,R)-2c, rather than by a biased propeller-
shaped helicity in 1b and 1c, even though a helical confor-
mation was predicted for 1 as the most stable structure by a
conformational search (Fig. 8). We indeed found remarkable
changes in the CD spectrum of (R,R,R)-1b upon gradual
addition of the achiral ditopic guest 4, in which multiple bisig-
nated Cotton effects emerged in the absorption region of 1b,
and which were substantially distinct from the original Cotton
effects of (R,R,R)-1b itself (Fig. 10a). Also, we found a quite
similar and significant change in the molar CDs when the
guest was mixed with (R,R,R,R,R,R)-1c, which possesses the
local point chirality in every blade (Fig. 10b). Molar CDs of 1b
and 1c in the presence or absence of a ditopic guest are sum-
marized in Table 1 along with those of 2. During complexa-
tion, we observed a hypsochromic shift from 368 to 357 nm for
the absorption maximum of 1b (366 to 357 nm for 1c) in UV-
vis spectroscopy, which indicated that blades relating to the
longest diametrical π-conjugation were induced to prefer a
benzoyl groups of 1 in solution. The UV-vis spectra of 1
(
showed typical absorptions for HPEBs consisting of
a
maximum (367 nm for 1a, 368 nm for 1b, and 366 nm for 1c)
and a shoulder (around at 390 nm) (Fig. 9a), both of which
underwent bathochromic shifts compared to the corres-
ponding absorptions of the parent HPEB (350 nm and 370(sh)
7
f
7c,d
nm, respectively) due to para-substitution in the blade.
As
in the case for 2, the similarity in absorption suggested that a
common structure was present in solution for HPEBs 1,
regardless of the bulkiness of an auxiliary on the amide
nitrogen.
The CD spectra of (R,R,R)-1b and (R,R,R,R,R,R)-1c were
similar with regard to the shape of the two major negatively-
signed Cotton effects, and their molar CDs increased nega-
tively with an increase in the number of chiral auxiliaries
[λext(Δε) around 270 nm (−5 for 1b and −10 for 1c) and around
3
60 nm (−3 for 1b and −6 for 1c)] (Fig. 9b). The spectral shape
was different than those of double-bladed (R)-2b or single-
bladed (R)-3 (Fig. 5b), and therefore the spectral intensities
were not simply tripled or sextupled compared to those of
(
R)-2b or (R)-3 due to the increase in associated chromophores.
a,b
Table 1 Molar CDs of 1, 2 and 3, in the presence or absence of a
ditopic guest, measured in CH
2 2
Cl at 293 K
λext/nm (Δε)
(
(
(
(
R,R,R)-1b
358 (−3), 321 (−2), 269 (−5)
R,R,R,R,R,R)-1c 362 (−6), 335 (−4), 268 (−10)
R,R,R)-1b
R,R,R,R,R,R)-1c 389 (+12), 361 (+11), 316 (−41), 282 (+10), 256 (−29)
a
386 (+6), 367 (+6), 315 (−20), 283 (+3), 254 (−10)
a
(
R)-2b
344 (+1), 290 (−6)
349 (+1), 287 (−10)
(R,R)-2c
b
2
a
341 (+2), 286 (−6)
a
(
R)-2b
331 (+5), 285 (−14)
a
(R,R)-2c
326 (+16), 283 (−40)
(
R)-3
291 (−5)
Fig. 9 (a) UV-vis and (b) CD spectra of (R,R,R)-1b (thin line) and (R,R,R,
R,R,R)-1c (thick line), measured in CH Cl at room temperature.
a
b
2
2
In the presence of 4 (4 equiv.). In the presence of (R,R)-5 (4 equiv.).
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7
a,b,18,19
twisting and/or strained conformation in the complex.
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Thus, we considered that these complexation-induced spectral
changes were the result of a chiroptical molecular propeller
induced in the 1 : 3 complexes (R,R,R)-1b·4
c·4 (Scheme 2).
3
and (R,R,R,R,R,R)-
1
3
2009, 10169; (g) E. Gagnon, T. Maris and J. D. Wuest, Org.
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Conclusions
We have demonstrated a method for constructing a chiroptical
molecular propeller based on the HPEB framework. The
important issue is how to force all of the blades to twist in a
conrotatory manner and to prefer a particular sense of the pro-
peller arrangements with (M)- or (P)-helicity. We focused on
three pairs of neighboring blades and regarded the HPEB
framework as a threefold double-bladed substructure. By
pairing two blades, a local point chirality was transmitted to
the double-bladed dynamic helicity and acted as a chiral
handle to control the helical preference, while such a local
chirality did not seem to exert any helical influence on the
dynamic helicity by itself due to the high flexibility of each
blade.
We found a helical conformation with neighboring blades
twisting in a conrotatory manner with the attachment of a suit-
able tertiary amide to each of the blades. The tertiary amide
nitrogen was modified with benzoyl and a series of alkyl sub-
2
stituents [Me, CH (cHex), or (R)-CH*Me(cHex)]. The chiral
auxiliary failed to induce a preference in dynamic helicity by
itself (weak chiral communication between blades). The
benzoyl groups in neighboring blades provided a binding site
for capturing a ditopic guest through hydrogen bonds, to give
a threefold supramolecular cyclic structure. The point chirality
6 R. Katoono, H. Kawai, K. Fujiwara and T. Suzuki, Chem.
Commun., 2008, 4906.
7 (a) K. Sakajiri, T. Sugisaki, K. Moriya and S. Kutsumizu,
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A. Villinger and P. Langer, Org. Lett., 2011, 13, 1618;
(
R) acted as a chiral handle to control the propeller-shaped
helicity only when HPEB was in a complexed state (complexa-
tion-induced chiral communication between blades), where
two neighboring blades twisted in a conrotatory manner
preferred a particular handedness, leading to a C -symmetric
3
chiroptical molecular propeller in the HPEB framework.
(
e) B. Traber, J. J. Wolff, F. Rominger, T. Oeser, R. Gleiter,
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1
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higher energy level (+2.78 kJ mol− ) (Fig. S2b†). This
small difference in energy between diastereomers with
(P)- or (M)-helicity might reflect weak chiral communi-
cation between blades in the absence of a guest, as
shown by small positively-signed Cotton effects around
at 350 nm (Δε < +1) in the CD spectra of (R)-2b and
(R,R)-2c. Thus, we concluded that these small effects
around at 350 nm were a part of bisignated Cotton
effects due to a slightly-preferred sense of the double-
bladed dynamic helicity.
1
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1
1
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2
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tives,
in addition to a propeller arrangement.
.
18 J. A. Marsden, J. J. Miller, L. D. Shirtcliff and M. M. Haley,
1
5 Almost the same helical structure was found for a model
J. Am. Chem. Soc., 2005, 127, 2464.
substructure (R)-2b′ [X = Me, Y = (R)-C*HMe(cHex)], 19 Upon complexation of 1a (7.2 × 10− M) with (R,R)-5 (12
5
which indicated that a double-bladed substructure seems
to prefer the helical conformation in solution as well as
in the crystal form, regardless of the bulkiness of an
auxiliary on the amide nitrogen: a helical conformation
with (P)-helicity was the most energy-minimized structure
equiv.), we observed a hypsochromic shift in the absorp-
tion of 1a (367 to 364 nm) similar to those induced for
(R,R,R)-1b or (R,R,R,R,R,R)-1c, while in the CD spectra no
significant change was induced. These results indicated
that 1a [X = Y = Me] formed a complex with (R,R)-5 through
hydrogen bonds, however any preference for a particular
sense was not induced.
(Fig. S2a†), and interestingly, another helical confor-
mation with inversed helicity was found at a slightly
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Org. Biomol. Chem.